Use python3 as the interpreter instead of python (ie v2)

This fixes lintian warning "unusual-interpreter".

Part of the Debian edition of FreeCAD since 2023.

src/Mod/Test/testmakeWireString.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 # ***************************************************************************
 # *   Copyright (c) 2013 WandererFan <wandererfan@gmail.com>                *

src/Mod/Test/unittestgui.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 """
 GUI framework and application for use with Python unit testing framework.
 Execute tests written using the framework provided by the 'unittest' module.